1. Field of the Invention
The present invention relates to a rewritable optical information recording medium capable of reproducing, erasing, and recording information by means of radiation beam incident on the recording medium.
2. Related Arts
Recently, various kinds of phase change type optical information recording media have been proposed. A typical phase change optical information recording medium has a substrate, a lower protection layer, a recording layer, an upper protection layer, and a reflection layer are formed on the substrate in that order. The crystal phase of the recording layer changes between a crystalline phase and an amorphous phase in response to irradiation of a beam, so that the reflectance of the recording layer changes, enabling the recording medium to rewrite information. However, thus rewritable recording medium must satisfy the same standard requirements as those of conventional optical recording media for reproducing only such as the Compact Disc (CD), the Laser Disk (LD) and the like, and this calls for an initial reflectance of 65% or more. The initial reflectance means a reflectance of a non-recorded portion of the recording medium.
To increase the initial reflectance, the inventor of the present invention proposed a rewritable recording medium in JP-A-5-205316. The proposed rewritable recording medium has a lower protection stack composed of several transparent layers having a different refractive index, respectively. Each thickness of the transparent layers is controlled to increase the initial reflectance of the recording medium. However, to obtain a sufficient initial reflectance, it is necessary to pile many transparent layers for the lower protection layer. As a result, the total thickness of the recording medium is increased, thereby being likely to produce cracks therein. In addition, the manufacturing cost of the recording medium is increased.